This is the U.S. National Phase under 35 U.S.C. xc2xa7 371 of International Application PCT/RU99/00155, filed May 13, 1999, which claims priority of RU/98109625, filed May 13, 1998, RU/98109078, filed May 13, 1998, and RU98120202, filed Nov. 6, 1998.
This invention relates to apparatus and method of forming of thin cantilevers for use in atomic force microscopes and other microscope systems.
The scanning tunneling microscope (hereafter abbreviated STM) is a probe instrument that is able to form images of solid surfaces with high linear and spatial resolutions. This is achieved by using of ultrasharp tip (probe), that is moved at small distances above the surfaces, by registration of the tunneling currents passing between the probe and the surfaces.
The atomic force microscope (hereafter abbreviated AFM) is a probe instrument that includes a cantilever containing a holder, a lever, and a tip probe, the probe placing on the lever and having atomic sizes at least at one dimension. The cantilever moves over solid surfaces and gives their images by registration of various forces involved (van der Waals, electrostatic, magnetic, etc, ones).
In this instrument, the measurement sensitivity and the quality of the images depend decisively on characteristics and parameters of the whole cantilever, as well as of the probe and, first of all, on its detailed shape.
It is known a cantilever (xe2x80x9cstylusxe2x80x9d) for AFM where a lever and a monolithically connected with it a tip probe are formed of an amorphous insulator such as silicon nitride, the probe having a shape of an inverted tetrahedral pyramid with angles at its apex about 110xc2x0 [1]. Such probes are not suitable for investigations of coarse irregular surfaces.
It is known a cantilever for AFM containing a single-crystalline silicon tip probe on a singlecrystalline silicon lever. In such a cantilever the probe has a conical shape at all its length, including its top part, the angle at its top being 20-30xc2x0, see FIG. 1 [2]. Such a shape limits possibilities to use the cantilevers for detailed studies of microelectronic structures, e.g., grooves with vertical walls [3-5]: in such studies it is important, as a rule, to investigate simultaneously a morphology of both the vertical walls and the bottom of the grooves. xe2x80x9cDead zonesxe2x80x9d are formed near bottom angles of the grooves, thus, the conical probes are not suitable for the studies (FIG. 2).
At a first glance, it is possible to study the grooves having vertical walls if use small-diameter cylindrical probes. However, the resolving power of such probes and a possibility to approach by such probes closely to the walls are limited because, as the diameter of the probe is decreased, its vibration stability is catastrophically deteriorated.
The problem of the vibration of the probes can be solved if the cylindrical part of the probe is combined with a massive basis, e.i., if the probe has a xe2x80x9csteppedxe2x80x9d shape. Such a shape can be provided by a crystal (xe2x80x9cwhiskerxe2x80x9d) growing process [6] according to the vapor-liquid-solid (VLS) mechanism, as it was described in [7], if the lower part (a xe2x80x9cbasisxe2x80x9d) of the whisker, that serves for forming of the probe, is grown at higher temperatures, while the upper part is grown at lower temperatures. However, such an approach suffer from the fact that, at the lower temperatures of the whisker growth, the crystallographic quality of the whiskers deteriorate so that the strength of the upper part of the probes can decrease that is non-desirable for such applications as AFM.
It is known a method for producing ultrafine silicon probes for the AFM/STM by etching of microstructures prepared from single-crystalline silicon by photolithographic procedures [8]. Tip shafts (xe2x80x9cprobesxe2x80x9d) are producing by reactive ion etching. Something similar to a base is formed at the lower part of the probe in order to eliminate vibrations. However, the proposed method does not allow to achieve required/optimal relationships of the upper (cylindrical tip) and the lower (thickened basis) parts of the probes, that is confirmed by drawings given in the patent [8].
Good relationships between the upper and the lower parts of the probes are achieved in [9], see FIG. 3, where the step-shaped probe is formed by focused ion beams. However, the method is complicated in implementation and is expensive.
In this invention, we propose a more simple and cheap method for production of the step-shaped probes. The method is based on the crystalline whiskers growing by the vapor-liquid-solid (VLS) process. Our method uses a special silicon-on-insulator (SOI) structure that allows to fabricate silicon cantilevers oriented along the crystallographic plane (111) necessary for growing of whiskers according to the VLS mechanism.
It is known a method for preparation of cantilevers with using of a silicon-on-insulator (SOI) structure where an oxide interposed between two silicon wafers is used as a xe2x80x9cstopxe2x80x9d layer at etching procedures [10]. In the method a holder and a layer of the cantilever are formed simultaneously with them. The composite SOI structure contains a layer of which the lever and the probe is subsequently formed and which is easily to treat anisotropically in a wet etch. In particular, a version is considered when the probe is formed of the silicon wafer oriented along the crystallographic plane (100).
The method [10] is illustrated in FIG. 4.
On the contrary, a wafer oriented along the crystallography plane (111) is used in the present invention. It is known, however, that such an orientation is hardly etched even at plasma (xe2x80x9cdryxe2x80x9d) etching and especially hardly at the wet etching that is used in patent [101. In the present invention, a composite SOI structure contains at least one wafer (111) which is specially used for formation of the lever and, subsequently, of the probe.
A drawback of the patent [10] and of most of other methods for preparation of cantilever consists in the fact there the formation of the holder, of the lever, and of the probe itself is combined in a complete technological cycle. This limits strongly possibilities for formation of special shapes of the probe.
The advantage of the present invention consists in the fact that, here, the formation of the probe itself it separated of the formations of the holder (xe2x80x9csupportxe2x80x9d) and of the lever. In such a technology the formation of the probe does not depend on the procedures of the formations of the holder and of the lever.
The present invention allows to fabricate the step-shaped probe shown in FIG. 5.
An additional advantage of the lever (111) used in the present invention is that the backside of the lever can be excellently polished by an anisotropic etching that improves the measurement quality.
The present invention determines the design of the probe formed on the cantilever (111), as well as a method for its preparation. An object of the present invention is to provide the cantilever (111) for use in a scanning probe microscopy (SPM) that includes both STM and AFM.
According to this invention, it is proposed to produce a cantilever for scanning probe devices that contains a silicon holder, a lever, and a probe monolithic with the lever and perpendicular to it, the lever being implemented of silicon layer oriented on (111), and the probe being implemented of whisker grown epitaxially to the lever.
The probe has a stepped shape, contains a lower part, that serves as a basis, and an upper part, the upper part being shifted to an edge of the lever relative to the center of the basis, and the both parts having circular and/or polygonal cross-sections. The basis and the upper part are co-axial, the upper part is epitaxial in respect to the basis. The diameter of the basis of the probe exceeds the diameter of the upper part at least 10 times, the upper part of the probe having the diameter less than 100 nanometers, the curvature radius of the probe being less than 10 nanometers, and the height of the probe being more than 1 micrometer.
The cantilever according to said above where the upper part of the probe has an expansion, the diameter of the expansion exceeding the diameter of another part of the upper part at least for 20%. And the side surface of the expansion can be faceted. After that, the expansion is followed by a contraction, top of the probe can be sharpened.
The holder is implemented of a silicon wafer coated by a layer of silicon dioxide, and the layer is coated by a silicon layer oriented along the crystallographic orientation (111). The holder represents a silicon wafer oriented along the crystallographic plane (111). The lever is formed of a body having Π-shape and/or V-shape. The lever contains a cavity longitudinal to its length. The lever contains a piezoresistive layer. An electric contact to the piezoresistive layer is implemented through a silicon film doped to the level p++.
A backside of the lever has a roughness less than 5 nanometers, and is coated by a light-reflecting material.
The cantilever contains at least two levers, at least one of them being placed on a side of the holder opposite to other ones.
The probe includes at least one n-nxe2x88x92, p-pxe2x88x92, or p-n junction.
The probe is coated by a stabilizing material, metal silicides being used as the stabilizing material.
The top of the probe is coated with a hard material and/or with a material lowering the electron work function. Diamond or silicon carbide are used as the hard material, whereas diamond or diamond-like carbon are used as the material lowering the electron work function.
A method for preparation of a cantilever for use in scanning probe devices is also proposed in the invention. The method includes a formation of a holder and of a lever from a silicon wafer and formation of a probe on the lever. A composite wafer is formed by bonding of two silicon wafers with an interposed oxide between them. The holder and the lever are formed from the composite wafer, and a silicon whisker probe epitaxial grown to the lever is formed. After that, the cantilever prepared is separated off the composite wafer.
At least one wafer oriented along the crystallographic plane (111) is used at the formation of the composite wafer.
After the formation of the composite wafer, a principal part of the first of the bonded wafers oriented along the plane (111) is mechanically and/or chemically removed so that a thin (111)-oriented layer is still remained. An oxide film is formed on silicon surfaces, and a part of the second of the bonded wafers is removed by etching so that a membrane of the remained first (111)-oriented layer is formed. Then, an oxide film is formed on silicon surfaces. The lever is formed of the membrane, and a whisker probe epitaxial grown to the lever is formed. Then, the cantilever prepared is separated off the composite wafer.
In another version of the method, after the formation of the composite wafer a principal part of the first of the bonded wafers oriented along the plane (111) is mechanically and/or chemically removed so that a thin (111)-oriented layer is still remained. A lever is formed from the silicon layer oriented along the plane (111), and an oxide film is formed on silicon surfaces. A part of the second of the bonded wafers is removed by etching so that a membrane of the remained first (111)-oriented layer is formed, and a whisker probe epitaxial grown to the lever is formed. Finally, the cantilever prepared is separated off the composite wafer.
In all the versions, the whisker probe can be sharpened before the separation of the cantilever.
A Π-shaped and/or V-shaped lever, as well as a lever with longitudinal cavity can be formed on the cantilevers. At the forming of the lever a piezoresistive layer and/or a p++ contacts on the surface of the cantilever are formed by ion implantation.
Another version of the method for preparation of a cantilever for use in scanning probe devices includes a formation of a holder, of a lever from a silicon wafer oriented along the crystallography plane (111). A whisker probe epitaxially grown to the lever is formed and, after that, the cantilever prepared is separated off the composite wafer.
According to the method, before the holder and the lever are formed, a silicon membrane is formed on a side of the silicon (111) wafer by means of electrostatically screened, inductively-coupled plasma containing gaseous fluorides. The whisker probe can be sharpened before the separation of the cantilever. An Π-shaped and/or V-shaped lever, and a lever with a longitudinal cavity is formed. At the forming of the lever a piezoresistive layer and/or a p++ contacts on the surface of the cantilever are formed by ion implantation.
In this invention also proposed a method for preparation of step-shaped silicon whisker probe that contain a lower part (basis) and upper part, by vapor-liquid-solid growth mechanism on a single-crystalline silicon substrate of crystallographic orientation (111) with using of metal solvent, and an expansion is formed at the upper part of the probe by changes of the growing temperature and/or concentration of silicon-containing gaseous compounds and/or transport agent in a vapor-gaseous mixture and/or pressure of the mixture and/or by adding of the metal solvent to the top of the whisker or its removing. Then, after the expansion of the upper part of the probe it can be contracted.
Also solidified globule of the alloy of silicon with a metal solvent that can be formed at the top of the whisker is removed by chemical etching, the whisker being sharpened. After that the probe formed is treated by an etch anisotropic in respect to silicon, faces on its surface being formed.
According to the present invention proposed a method for preparation of a step-shaped silicon whisker probe that contains a lower part serving as a basis, and upper part by growing of the whisker according to the vapor-liquid-solid mechanism on a single-crystalline silicon substrate of the crystallography orientation (111) with of a metal solvent, where a metal solvent represents a liquid alloy consisting of at least two metals. In so doing, the metals differ each of other in their vapor pressures more then by one order of magnitude.